# Questions tagged [black-holes]

A black hole is a volume from which matter cannot escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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### What does an external observer see if you shine a laser pointer while crossing an event horizon?

If I understand correctly, the event horizon of a black hole is the boundary beyond which we cannot perceive or describe any events, and all objects appear to freeze when falling in until their light ...
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### Particle falling into a Kerr black hole

Let's say that a particle starts a radial free fall towards a Kerr black hole with zero initial energy at $r\rightarrow\infty$. The initial angular momentum of the particle is zero ($p_\phi = 0)$. ...
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### What do I see looking forward inside the black hole? [closed]

I have read this question: Future light cones inside black hole What happens to light that falls into a black hole? What would be the view like from inside a black hole looking towards the event ...
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### River model of a Kerr black hole: What does it mean for the spatial metric to be sheared?

I am currently trying to understand the river model for a Kerr black hole. There is one concept I don't grasp yet. What does it mean for the spatial metric to be sheared in the given context. I am ...
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### Komar mass derivation

The Komar mass, a conserved quantity associated with an asymptotically time-like Killing vector, for stationary spacetime is $$M_K=\frac{-1}{8\pi}\int\int\nabla^{\mu}\xi^{\nu}_{(t)}dS_{\mu\nu},$$ ...
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### Entangled particles separated by event horizon

When referring to Hawking radiation I often see the example of a particle-antiparticle pair which become separated by the event horizon boundary with one half of the pair remaining outside and the ...
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### how to create a kinetic energy kugelblitz in theory? [duplicate]

Black holes form when enough mass gets packed so close together that its gravity gets so strong that even light cannot escape it. Mass and energy are equivalent, so theoretically it is possible to ...
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### Does Hawking radiation never happen because a black hole takes an infinite time to form? [duplicate]

Consider the classical solution of a black hole and imagine some matter crossing the event horizontal so that it cannot escape the black hole. I don't remember in the case of a rotating black hole, ...
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### Calculation of surface gravity of acoustic black hole

I have been trying to calculate the surface gravity of analogue black hole formed in a classical fluid, following Visser (https://arxiv.org/abs/gr-qc/9712010, p19, section 9.1). The line element in ...
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### Tachyon black hole, would it repel gravitationally? [closed]

This question is not about tachyons entering normal matter BHs, nor is it about what would happen to tachyons in BHs. It is about a BH made of tachyons. Tachyons on this site are considered quasi ...
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### Could a perfect sphere contain a black hole?

Imagine we have a small black hole, with a mass somewhere around a mountain or small planet. Since it is a single point, it should have an even amount of pull across all matter at the same distance, I ...
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### Can black holes prevent heat death?

I recently learnt that black holes possess among the highest amounts of entropy in the universe. I might have stretched the idea preposterously far, but is it possible that this might be one of the ...
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### Researching Black Hole Spin/Mass

I'm a high school student investigating how one can predict the mass and spin of the product of a binary black hole system. I'm fluent in Calculus (Single Variable), Number Theory, and Graph Theory, ...
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### Escape a black hole by tunneling faster then the speed of light [closed]

I do understand that nothing can escape a BH (not particles, no information), and this question is not about why you cannot escape. This question is not about Hawking radiation, and I do understand ...
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### If I throw a ball at a black hole, will the ball exceed the speed of light when it reaches the event horizon? [duplicate]

The escape velocity of a black hole at the event horizon is the speed of light, this means an object that is dropped from the top of the black hole's gravitational well will reach the speed of light (...
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### ER bridge in black hole merger

Does an ER bridge between two black holes mean they are "entangled" and what happens to the ER bridge if a merger happened?
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### How big is the role of black hole spin in forming its mass?

Excuse me if i'm saying something weird, but as far as i know moving faster means increasing mass according to relativity theory, right? If you make some star spin really really fast throwing ...
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### Magnetic field of falling charge into black hole

Lets think about a point charge is falling into black hole. I know that electric field can be get out from the black hole. Then how about magnetic field? What happens to magnetic field since the ...
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### Why aren't black holes antimatter fountains?

As I understand it, nothing can escape a black hole because the only worldlines leading 'out' from the event horizon point backwards in time. As I understand it, antimatter is time-reversed ordinary ...
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### Kretschmann scalar is discontinuous

I am studying this paper: "Non-singular rotating black hole with a time delay in the center" by T. de Lorenzo et al. In this paper authors calculate a new metric for a regular, rotating black hole. ...
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### What happens to the accretion disk when two black holes merge

I'm aware that accretion disks around black holes are formed from the swirling mass of matter that is slowly being stripped of its atoms, but what happens to it when two black holes merge? I was ...
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### If, for outside observers, infalling matter never enters the black hole, what happens to the matter's gravity?

It is my understanding that observers outside the black hole wouldn't ever see some infalling object enter the black hole, as the object's clock would be running so slow in the observer's reference ...
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### Growth of apparant horizons and null convergence condition

An apparent horizon in general relativity is a surface where all null vectors are pointing "inwards", i.e. it is the location of a marginally outer trapped surface (for a review see here). It is well ...
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### How does light energy increase when passing through the ergo sphere of a black hole?

I watched a video recently about using the ergosphere of a spinning black hole as a source of virtually infinite energy by surrounding it with mirrors and shining electromagnetic waves in there. In ...
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### Bob and Alice - can Alice save Bob from the black hole with a very long pool skimmer?

If Bob is in free-fall but close to the event horizon, and appears frozen to Alice, who is stationary at a much farther distance, and Alice were to extend a very, very long pool net and try to scoop ...
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### Do the logarithmic corrections to black hole entropy imply corrections to their energy or temperature?

Could somebody help guide the thinking in this situation? Do the corrections to entropy $S$, like those of https://arxiv.org/abs/hep-th/0111001, affect the temperature of a black hole, or its mass, ...
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### Wormhole between two distant point in space, where are the two ends?

I'm struggling with the wormhole solution of the Einstein equation. I can't understand how it may connect two distant points in the universe. It looks to me totally misinterpreted and I would like to ...
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### Events inside a Blackhole [duplicate]

I am trying to understand the concept of event horizon and what really the word "event" means and have some questions. I apologize if the questions seem stupid or vague as I am not a physics student ...
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### Can light of certain wavelengths stably orbit a black hole at the photon sphere?

I've recently learned that for a black hole, its photon sphere is an unstable orbit. Light either manages to escape from or fall into the event horizon, depending on how far away from the edge of the ...
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### How does time dilation affect power transmission (and V, A, W, Hz)?

How is the generation and consumption of electricity affected under the influence of time dilation? (For instance, a colony ship under the influence of a black hole) To get some hard numbers into ...
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### Is there any practical way we can verify Bob and Alice, Whitehole and Leonard Susskind's famous Hologram theories?

Alice and Bob analogy near Blackhole is a well known hypothetical theory that says suppose Alice and Bob are in a spacecraft near the Supermassive Blackhole located at the center of the Milkyway ...
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### Would looking out from inside or near a black hole be unimaginably bright? [duplicate]

I have the following assumption based on limited knowledge. A black hole appears dark to us because any light that would be emitted from it, and any light that passes nearby, is caught by the gravity ...
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### Would a “naked singularity” and a spinning black hole behave the same?

So I've been doing a lot of research on spinning black holes and fumbled upon the concept of a "naked singularity" where a gravitational singularity exists without an event horizon. Ignoring the ...
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### If the Universe were a white hole, how would inflation fit in

So, I was thinking about white holes and one theory in particular about white holes, and that is that white holes, the theoretical opposite of black holes, are each "Big Bangs". That is to say on the ...
Does the Weyl curvature tensor $C$ of the black hole singularity in the conformal cyclic cosmology diverge to infinity unlike the Big Bang (C = 0)?